1. Field of the Invention
The present invention relates to insulating material and an electrical heating unit employing same, which are used in heating apparatuses such as various types of industrial furnaces and experimental furnaces, and a manufacturing method therefor.
2. Description of the Related Art
Insulating material of vacuum formed ceramic fiber has a high insulating performance, is lightweight, and can also be shaped into arbitrary forms. Moreover, this insulating material has sufficient strength and is easy to handle; secondary machining is also easy. This material has been used effectively for improving the loss of heat energy from furnace walls. Electrical heating units using this insulating material are also known. For example, U.S. Pat. No. 3,500,444 discloses a technique for economically manufacturing an electrical heating unit by embedding a heating element near one surface of such insulating material. Also, U.S. Pat. No. 4575619 discloses a grooved electrical heating unit, comprising a serpentine heating element, with improved thermal radiation characteristics.
These electrical heating units have the advantages that they can be formed into arbitrary shapes and they have the same superior insulating performance as the above-mentioned insulating material itself, of which they are formed. Furthermore, they have the advantage of having sufficient mechanical strength that they can alone constitute furnace walls. Consequently, because it is easy to assemble a furnace by using these in an appropriate combination, it becomes possible to greatly reduce labor in constructing a furnace and thus contributing greatly to the cost reductions of energy conserving furnaces.
Since then, however, the industrial sector has become more and more strict about reducing environmental loads because of the increased attention to global environmental problems. These problems have become manifest and their resolution is an issue for all people. Making furnaces much more energy efficient is therefore a significant task.
Meanwhile, the insatiable pursuit of improved insulating performance has drawn attention to the properties of microporous material such as silica aerogel, especially with its micro-spherical structure with a minute closed vacancy, smaller than the mean free path of gas. Thus, so-called microporous insulating material has been developed which has an ultimate insulating performance, i.e. ability to theoretically eliminate the convective heat transfer between voids in the insulating material.
Related technologies include U.S. Pat. No. 3,869,334 which shows how a high performance insulating material, which can be handled as an ordinary insulating material, is attained by enclosing silica aerogels in a bag made of fiberglass cloth and pressure forming same into a flat panel. The insulating performance is known to be much better than that of a vacuum formed ceramic fiber. As a result of achievements in manufacturing technologies, recently silica aerogel materials formed directly into boards are also available because the strength thereof has been improved by blending the aerogel with refractory fiber material or the like, instead of enclosing it in the abovementioned bag.
These available microporous insulating materials comprising silica aerogels or the like are essentially low in strength because of the characteristic structure of silica aerogel as a constitutional element; specifically, a microspherical shell containing a hollow in it. In addition, available thickness is also limited, so it is not possible to construct the furnace walls with these alone. Hence the use of these materials as insulating material for furnaces is limited to backup material or intermediate layers of lining material. While use in such forms can ensure energy conservation, this usage has the problems of increasing the labor in constructing the furnaces and adding up costs. Also, especially when in board form, these materials are easily damaged or broken during construction and much expensive material is wasted.